Bisphenols are raw materials for the production of polycondensation materials such as epoxy molding compounds, polyether sulphones, polyether ketones or polycarbonates. Bisphenols are generally produced by reacting phenol or substituted derivatives thereof with suitable ketones in the presence of acidic catalyst and with separation of water. The industrially most significant bisphenol is bisphenol A (BPA), produced from phenol and acetone. Bisphenols derived from cyclic alkanes, for example the condensation product of phenol and 3,3,5-trimethylcylohexanone (BP-TMC) are also very important in the production of polycarbonates.
Homogeneously dissolved acids such as hydrogen chloride or heterogeneous acid fixed-bed catalysts such as sulphonated cross-linked polystyrene resins (acid ion exchangers) are used as catalysts for the production of bisphenols. While the use of heterogeneous catalysts is to be preferred from certain viewpoints to the use of homogeneous catalysts, it may be found in EP-A 995 737 that inadequate reaction and selectivity is achieved with this type of catalyst for certain ketones. Therefore, the use of strong acids such as hydrochloric acid as a catalyst is to be preferred for a large number of ketones, in particular cyclic ketones. To further increase the ketone reaction and to raise the selectivity of the reaction, sulphur-containing organic compounds such as alkyl mercaptans, thiocarboxylic acids or dialkylsulphides, as described in U.S. Pat. No. 5,210,328, are used as cocatalysts. The use of specific alkane thiols is to be derived from U.S. Pat. No. 5,336,812, while EP-A 995 737 proposes the use of alkyl mercaptans with 1 to 12 carbon atoms.
Mixtures containing the desired bisphenol, isomers, intermediates and secondary products of the desired product, as well as unreacted raw materials and water, and the catalyst and cocatalyst used and optionally the reaction products thereof with the components of the reaction system are generally obtained as a result of the reaction of phenols and ketones under the above-mentioned conditions. To obtain bisphenol products of suitable quality for producing high-grade polymer materials, it is necessary to separate these by-products and reaction components as completely as possible from the reaction product bisphenol. For this purpose a combination of various standard purifying operations such as crystallisation, extraction or distillation are usually carried out. Various problems can occur in this process. The bisphenols obtained are thus generally thermally unstable in particular in the presence of catalytically active compounds, acids or bases. This is particularly problematical when using homogeneously distributed acids as catalysts which remain in the product mixture. It is disadvantageous in the neutralization with bases described in EP-A 995 737 or in the extraction of the acids by addition of water proposed in EP-A 679 151 that large quantities of organically loaded waste water are produced by these measures and have to be processed in expensive purification operations. It is also difficult in a procedure of this kind to carry out the reaction continuously with separation of the catalyst. It is also not ensured in a procedure of this type that the sulphur-containing cocatalyst used is also substantially separated from the reaction mixture. Residues of the cocatalyst in the purified bisphenol impair the suitability thereof for producing high-grade polycondensation materials.
A further problem in the production of bisphenols from phenols and in particular from ketones with more than 5 carbon atoms is that the reaction mixtures with high proportions of produced bisphenol can become solid owing to crystallisation of the product, so efficient continuous reaction control and separation of the catalyst is no longer possible. Subsequent melting of the reaction mixture or carrying out the reaction at elevated temperatures to avoid this problem leads to undesired side reactions and reduced selectivities. Reaction control with a high phenol excess or incomplete ketone reaction to avoid a high bisphenol product concentration is disadvantageous, as the space-time yield is thus reduced. Excess phenol and ketone also have to be separated when working up the reaction product. It is proposed in EP-A 995 737 to initially allow phenol and ketone to react in a prereaction until at least 90 mol % of the ketone has reacted and then to add a further quantity of phenol and/or aromatic hydrocarbon to the reaction mixture. A procedure of this type is awkward, does not solve the problem of separating the catalyst and may even introduce a further material into the method which later has to be separated.
The object of the invention is to provide a method for producing bisphenols with acid catalysis in the presence of a sulphur-containing cocatalyst which has a high space-time yield and high selectivity and supplies a product which can be fed to further purification without further expensive processing steps.